1. Field of the Invention
The present invention relates To spark plugs for use in internal combustion engines. More particularly, the present invention relates to a method of making spark plugs which include wear-resistant electrode tips made from a co-extruded composite material, and to spark plugs incorporating such wear-resistant electrode tips.
2. Decription of the Background Art
Spark plugs are widely used to ignite fuel in internal combustion engines. Spark plug electrodes are subject to intense heat, and to a highly corrosive environment, generated by the exploding air/fuel mixture, To improve durability and erosion resistance, spark plug electrodes must be able to withstand the high temperature and corrosive environment resulting from the chemical reaction products between air, fuel, and fuel additives within a combustion chamber. The same chemical and thermal stresses also affect the interface between the ground electrode and the metal spark plug shell to which the ground electrode is bonded. Where this interface does not consist of a strong bond, these stresses can reduce spark plug performance or even cause the spark plug to fail.
Society of Automotive Engineers paper No. SAEJ312 describes the specification for automotive gasoline used as a fuel in the United States. The gasoline consists of blends of hydrocarbons derived from petroleum: 50-80 percent saturates, 0-15 percent olefins, and 15-40 percent aromatics. Leaded gasoline contains about 0.10 grams of lead per gallon of fuel (0.026 g Pb/liter), and 0.15 percent sulfur. In unleaded gasoline there is about 0.05 grams of lead per gallon (0.013 g Pb/l), 0.1 percent sulfur, and 0.005 g phosphorous per gallon (0.0013 g P/liter).
In addition, there are a number of additives incorporated into gasoline for various reasons. For example, tetramethyllead (TML) and tetraethyllead (TEL) are added as antiknock agents. Carboxylic acid compounds such as acetic acid are added as lead extenders. Aromatic amines and phenols are added as antioxidants. Organic bromine and/or chlorine compounds are added as scavengers and deposit modifiers. Phosphors and boron-containing compounds are added to reduce surface ignition, preignition, and as engine scavengers. Metal deactivators are added to reduce oxidative deterioration of fiel by metals, such as Cu, Co, V, Mn, Fe, Cr and Pb. In addition, carboxylic acids, alcohols, amines, sulfonates, and phosphoric acid salts of amines are used as nist-inhibiting additives.
Another factor which places a stress on spark plugs in the combustion chamber environment is the use of Exhaust, Gas Recirculation (EGR) back into the combustion chamber, to cool the combustion charge and to improve emissions, particularly by reducing oxides of nitrogen.
The manufacture of copper (Cu) and nickel (Ni) electrodes for spark plugs is a proven art and has been accomplished in various ways. For instance, U.S. Pat. No. 3,803,892 describes a method of producing extruded copper and nickel electrodes from a flat plate of the two materials. U.S. Pat. No. 3,548,472 discloses a method of cold-forming an outer nickel cup-shaped sleeve in several steps, inserting a piece of copper wire into the cup, and then lightly pressing the two materials together. U.S. Pat. No. 3,857,145 discloses a process for making a spark plug center electrode in which a central copper core is inserted into a nickel member and attached thereto by a collar portion, to assure that an electrical flow path is produced.
U.S. Pat. No. 4,093,887 to Corbach et al. discloses a design for a spark plug having a center electrode made of a composite material. In the design for the composite spark plug electrode as taught by this reference, the electrode is about 2.4 mm in diameter, and includes an outer cylindrical metal jacket, which may be made of nickel, a nickel alloy, or a material based on chromium or cobalt. Inside this outer metal jacket, according To the reference, a matrix material of high conductivity, such as copper or a copper alloy, has a plurality of parallel strands embedded therein. The embedded strands are each approximately 0.3 mm in diameter, and are formed from the same material as the outer jacket. The strands are preferred to be seven in number, are placed so that they do not touch each other, and are arranged so as to be distributed essentially uniformly over the cross-section of the matrix material. This reference does not specifically teach or suggest The use of a wear-resistant electrode tip, but rather, teaches that the entire center electrode be made of the described composite material.
The use of certain types of embedded and/or welded-on spark plug electrode tips, which are more wear-resistant than the main body of the electrode, is also known. In recent years, the practice of adding these wear-resistant tips to spark plug electrodes has become favored in the art. Such spark plug electrode tips may be added to the center electrode, to the side electrode, or to both of the center and side electrodes. Such wear-resistant electrode tips are made tougher and more erosion resistant than the balance of the electrodes, and since the wear-resistant electrode tips provide the points where the spark crosses over between the electrodes, they are among the most critical working parts of a spark plug. Sometimes these electrode tips are mechanically flattened out or xe2x80x98coinedxe2x80x99, during or subsequent to the attachment thereof to the base electrode, to cover a larger surface area than would otherwise be the case.
Some illustrative examples of patents relating to various wear-resistant spark plug electrode tips, and to spark plugs including such electrode tips may be found in U.S. Pat. Nos. 4,324,588, 4,810,220, 4,684,352, 4,810,220, 4,840,594, 5,179,313, 5,456,624, 5,558,575, 5,574,329, and 5,869,921.
Some of the known wear-resistant spark plug electrode tips incorporate platinum and/or other noble metals, because of their excellent resistance to oxidation and erosion under exposure to a combustion chamber environment. However, platinum is a very expensive raw material, as are the other noble metals, and it is therefore advantageous to strictly control the amount of noble metal which is incorporated into each spark plug.
In addition, the welding together of two dissimilar metals may result in a mismatch of the relative coefficient of thermal linear expansion of each metal. Under high thermal stress, this mismatch can lead to weakening or fracture of the bond between the electrode and the tip, and may even lead to physical separation of the noble metal and base metal.
U.S. Pat. No. 5,510,667 to Loffler et al. discloses a design for a spark plug which incorporates a reinforced electrode tip made of a platinum-nickel fiber composite material. The disclosed material, in this reference, may be a platinum matrix in which nickel fibers are embedded, or a nickel matrix in which platinum fibers are embedded. No specific number of embedded fibers is discussed in this reference, although the drawings appear to show a large number of fibers in the matrix. Alloys, which include platinum and another metal or metals, are not specifically disclosed in this reference.
The Loffler et al. ""667 patent cites to a German patent application number 2 508 490 as disclosing a suitable method of making a fiber composite material which is usable to pracice the invention thereof. The above-cited Gernan patent application also corresponds to Great Britain patent application number 1 528 514, filed Feb. 26, 1976. The method taught therein involves loosely placing solid wires inside hollow metal tubes formed from a dissimilar metal than that of the wires, and bundling multiple tubes and wires together, inside of a larger tubular jacket. The placement of the bundled tubes in a metal jacket is followed by cold plastic deformation of the jacket, tubes, and wires together, to produce a composite material. Different end products are obtained, depending on which materials are used for the component parts.
Society of Automotive Engineers Publication No. 1999-01-0796 discusses the advantages of using an alloy of iridiumn and 10% rhodium for a wear-resistant spark plug tip, and the fiber advantages of keeping the diameter of the central electrode small.
Although various designs for spark plugs having wear-resistant electrode tips are known, a need still exists in the art for a method of making a wear-resistant spark plug electrode tip, in which an amount of platinum, inidium, or other noble metal used is strictly controlled for maximum practical efficiency and cost control.
The present invention provides a method of making a spark plug electrode tip incorporating two or more co-extruded materials, and to a spark plug incorporating a tip made by the preferred method. A wear-resistant spark plug electrode tip according to the invention is preferred to be made in a post or rivet shape, and a rivet is most preferred.
Preferably, one of the materials used in fabricating the tip is a noble metal, which may be selected from the group consisting of platinum, iridium, and alloys which include one or both of these metals. In the wear-resistant electrode tip according to the invention, the noble metals or their alloys are preferred to be present in the form of one or more oriented strands of wire encased in, or evenly interspersed throughout a carrier or matrix metal. The matrix metal is preferred to be a nickel compound.
In the practice of the present invention, the material used for the matrix metal is selected to have a coeffcient of linear thermal expansion which is similar to that of the base metal of the electrode to which the tip is going to be attached. This matching of the matrix metal with the electrode base metal reduces or eliminates separation of the wear-resistant tip from the base electrode.
The oriented strands of wire are preferred to be disposed, within the electrode tip, so as to be parallel to a longitudinal axis thereof. The number of oriented strands of noble metal alloy is preferred to be between 1 and 20 strands.
A wear-resistant spark plug electrode tip according to the present invention may be attached to the center electrode of a spark plug, to the side electrode, or to both of the center and side electrodes. Optionally, in one method of practicing the invention, the tip, in any shape, may be flattened, or xe2x80x98coinedxe2x80x99, to increase the surface area thereof.
Accordingly, it is an object of the present invention to provide a method of making a spark plug which includes a wear-resistant electrode tip having a reduced content of a noble metal therein, and to a spark plug which is a product of the method.
It is another object of the present invention to provide a spark plug having at least one wear-resistant electrode tip attached to an electrode thereof, in which the electrode tip is formed from two dissimilar metals and includes at least one oriented strand of a metal compound comprising a noble metal therein, and wherein each oriented strand used is substantially parallel to a longitudinal axis of the electrode tip.
It is a firther object of the present invention to provide a particularly preferred embodiment of a spark plug, including a first wear-resistant electrode tip attached to a center electrode thereof and containing at least one oriented strand therein including a noble metal or a noble metal alloy, the spark plug further including a second wear-resistant electrode tip attached to a ground electrode thereof and containing at least one oriented strand therein including a noble metal or a noble metal alloy.
For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts.